The present invention relates to a directly detectable image sensor applicable to a document reading device of a fascimile terminal and others and a method of producing the same.
In the art of document reading devices, or document readers, such as a one intalled in a facsimile terminal, there has recently been developed a so-called directly detectable image sensor as disclosed in, for example, Japanese Laid-Open Patent Publication No. 60-134486 and a paper entitled "A STUDY ON STRUCTURE OF DIRECTLY DETECTABLE IMAGE SENSOR", Institute of Electronics and Communication Engineers of Japan, Report ED81-35. All of the sensors disclosed comprise a substrate made of glass, a light intercepting layer, an insulating layer, sensor elements and electrodes which are provided on the substrate with the intermediary of the light intercepting and insulating layers, and a transparent protective layer provided on the sensor elements and electrodes and implemented with a thin layer of glass. A document is laid on and held in close contact with the protective layer, so that the sensor elements may read images which are recorded in the document. Specifically, the light intercepting layer and electrodes are individually formed with windows in order to guide light, which is incident to the sensor via the substrate, toward the document, the sensor elements being sensitive to a reflection from the document.
A prerequisite with any of the prior art image sensors of the type described is that in order to attain a sufficient output for an MTF on the order of 0.6, the transparent protective layer be about 75 .mu.m thick, the windows be sized about 90 .mu.m.times.60 .mu.m, and the illuminance on the document surface be about 2500 1x. Under this condition, the the protective layer should be implemented with a glass sheet which is as thin as about 70 to 80 .mu.m. A problem with such a thin protective layer is that it deteriorate the sensor elements due to adhesion or an adhesive itself and is dynamically fragile to limit the yield. Another problem is that in the event of installation of such a sensor in a facsimile terminal, for example, a special consideration have to be given to external forces which may damage the sensor, resulting in an increase in cost. Furthermore, it is difficult for the image sensor to be provided with a high resolution such as 16 dots/mm since each of the windows should be as great as about 90 .mu.m.times.60.mu.m. Specifically, any of the prior art systems cannot achieve a resolution of 16 dots/mm unless the windows are halved in size and the protective layer is implemented with a glass film whose thickness is less than about 30 .mu.m. This would not only lower the sensor output but also aggravate the fragile nature of the sensor.